Compensation circuit for electronic photocomposition system

ABSTRACT

An electronic photocomposition system compensates for variations in the density of characters when the point sizes of the characters are changed. This is accomplished by causing the scanning beam that forms the individual character slices to also fill in the interstitial spaces between adjacent scanlines as the characters are enlarged.

United States Patent Inventor John C. Schira Princeton, NJ.

Appl. No. 677,324

Filed Oct. 23, 1967 Patented Apr. 6, 1971 Assignee RCA Corporation Priority June 12, 1967 Great Britain 27093/67 COMPENSATION CIRCUIT FOR ELECTRONIC PHOTOCOMPOSITION SYSTEM 7 Claims, 4 Drawing Figs.

US. Cl

Int. Cl

Field of Search Primary Examiner-John W. Caldwell Assistant ExaminerDavid L. Trafton Attorney-H. Christoffersen ABSTRACT: An electronic photocomposition system compensates for variations in the density of characters when the point sizes of the characters are changed. This is accomplished by causing the scanning beam that forms the individual character slices to also fill in the interstitial spaces between adjacent scanlines as the characters are enlarged.

DAIVE' 47671912 J M/700W iii Eff Gilt/561727! p use/44m? COMPENSATION CIRCUIT FOR ELECTRONIC PHOTOCOMPOSITION SYSTEM BACKGROUND OF THE INVENTION Mechanical and photographic techniques of composing type are relatively slow and the probability of significantly increasing the speed of such type composition appears to be slight. The successful transformation of type composition into an electronic art promises not only to greatly increase the speed of type composition but also to provide additional advantages. For example, in mechanical type composition systems, the point sizes of the characters can be changed only by substituting an entirely new font for each change in point size. Such substitutions require time as well as large storage areas to store the large number of fonts that are required for even relatively simple printing jobs.

SUMMARY OF THE INVENTION In an electronic photocomposition system, the light generated in scanning a display device, such as a cathode ray tube, is projected onto a photosensitive medium, such as photographic film, to record the characters formed on the display device. The characters are made up of a plurality of character slices that are created by a plurality of scanline segments that are unidirectional for characters of a predetermined point size. Means for changing the point sizes of the characters by altering the length of the scanline segments and the spacing between'adjacent scanlines is included in the electronic photocomposition system. To fill in the interstitial spaces between adjacent scanline segments, alternations that are orthogonal to said one direction are superimposed on the scanlines. The alternations exhibit an amplitude correspond ing to the point size of the characters generated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic block diagram of an electronic photocomposition system embodying the invention;

FIG. 2 comprises FIGS. 2a and 2b, which are graphical illustrations of the formation of a character in the system of FIG.

FIG. 3 comprises FIGS. 3a and 3b, which are graphical illustrations of the formation of a character when the point size of the character is changed; and

FIG. 4 is a graph illustrating the input-output characteristics of a modulator included in the system of FIG. 1 as well as the differences in the scanning pattern at different biasing points of the modulator.

vertical character slices that include scanline segments 30, ,such as those shown in the capital T in FIG. 2a. The black seg ments 30 comprise the portions of the scanline when the electron beam 14 is unblanked. Of course, in the cathode ray tube 12 itself, the segments 30 are actually light on a dark background whereas the segments 30 are shown dark on a white background in the drawing for illustrative purposes. Each scanline is deflected vertically upwardly from a baseline 32 to a terminating line 35 and then reset back to the baseline 32. This is accomplished by applying a sawtooth sweep signal derived from a sawtooth generator 34 t0 the vertical deflection coils 20. The sawtooth signal is applied through a potentiometer 36 having a plurality of positions I through P which determine the point sizes of the characters 22, as will be described in more detail subsequently. The output of the I potentiometer 36 is amplified in a driver amplifier 37 before DETAILED DESCRIPTION Referring now to FIG. 1, an electronic photocomposition system 10 embodying the invention is shown. Although the invention will be described in terms of a phototypesetting system, it is to be understood that the invention maybe embodied in any alphanumeric display system wherein the size of the symbols displayed is to be changed. The system I0 includes a display device, such as a cathode ray tube 12 having an electron scanning beam 14 that emanates from a cathode 16 in the electron gun section (not shown) of the tube 12. The electron scanning beam 14 is deflected by the main horizontal l8 and vertical 20 deflection coils that surround the neck of the cathode ray tube 12. It is apparent that electrostatic deflection may also be utilized in the tube 12. The scanning beam I4 creates a scanning spot 21 that forms the character images 22 in the phosphor on the face 24 of the tube I2. The light emanating from the phosphor in the tube 12 is focused by a lens 25 onto a photosensitive medium such as photographic film 26 that is supported by reels 28 in the focal plane of the lens 25.

While there are a variety of techniques of forming characters 22 on the face 24 of the device 12, a technique that is utilized to form characters of a high graphic quality is described. In this technique, each character 22 is built up by a plurality of application to the vertical deflection coils 20. In each vertical scan, the electron beam 14 is unblanked during the time that a black segment 30 is desired. This result is accomplished by unblanking signals applied to the cathode 16 of the tube 12 from an electronic data processor and controller 40 which may, for example, comprise a stored program computer. The processor and controller 40 also includes and adjunct memory for storing digital signals that produce the characters 22 in the various fonts desired. The processor 40 also supplies the command signal to trigger the sawtooth generator 34.

At the end of each vertical scanline, the electron beam 14 is stepped horizontally in a discrete step. This is accomplished by providing a digital counter 42 that is advanced by the processor and controller 40 to produce successively higher counts. The advance pulses are applied through a switch 43 having a plurality of positions P, through P that may correspond to the positions P through P in the potentiometer 36 for changing point sizes as will be described subsequently.

Each successively higher digital count is converted to an analogue signal of a successively higher magnitude in a digitalto-analogue converter (D/A or DACON) 44. The analogue signals from the DACON 44 are applied through a driver amplifier 46 to the horizontal deflection coils 18. Each analogue signal positions the scanning spot 21 to an invariable horizontal location on the tube 12. Thus as shown in FIG. 2a, each character is composed of a plurality of vertical segments 30. The segments 30 in actuality touch each other.

The photographic film 26 is composed of high-gamma film so that the light emanating from the spot 21 during the scanlines provides uniformly dense characters on the film 26. At the end of a line of characters 22, the reels 28 are driven by a drive motor 48 to move the film 26 to the next line to be printed. The processor and controller 40 provides the drive signal to activate the motor 48. Since the film 26 permanently records each segment 30, no refresh of the cathode ray tube 12 is needed. The cathode ray tube I2, the lens 25 and the film 26 are mounted in a lighttight cassette (not shown) having doors mounted thereon for accessibility.

In order to change the point size of the characters 22 produced, the length of the black segments 30 and the spacing between the scanlines are changed. Thus the characters 22 can be expanded or shrunk without distorting them, for a variety of point sizes. To change the height of the black segments 30, the potentiometer 36 is changed from the position P,, which produces the smallest amplitude sawtooth signal and hence the smallest point size character (e.g. point size 4), to any position up to the position P The position P produces the largest amplitude sawtooth signal and hence the largest character (e.g. point size I6). Similarly the horizontal spacing between scanlines is altered from the position P the closest spacing, in the switch 43, to any position up to the position P,,, the furthest spacing. The switch 43 position change 'is performed automatically by a point size information signal derived from the processor and controller 40. Thus the switch 43 is desirably an electronic switch. Each different position I through P actuates a different number of stages in the counter 42 to enlarge or contract the counter 42. The positions P through P in both the switch 43 and the potentiometer 36 maybe designed to provide corresponding and compatible changes in the point sizes of the characters 22.

When the point size is changed, the scanning spot 21 may not overlap in each succeeding segment the proper amount to provide uniform exposure of the film 26. Accordingly, there is included in the electronic photocomposition system 10, a compensation circuit 50 that includes an oscillator 52 that is coupled to an amplitude modulator 54. A control signal, which is the point size information signal, derived from the electronic data processor and controller 40 is also applied to the modulator 54 through the switch 43. The control signal specifies the point sizes of the character to be created and is selected to comprise a direct current biasing signal having a magnitude corresponding to this size. The amplitude modulator 54 may for example comprise a square law modulator having an input-output characteristic 56, as shown in FIG. 4. A diode modulator fulfills this requirement. The amplitudemodulated oscillatory signal derived from the modulator 54 is coupled through a driver amplifier 57 for application to an auxiliary horizontal deflection coil 59. The auxiliary deflection coil 59 is a low-inductance coil on the order of l microhenry in inductance value. The coil 59 may for example comprise a glass epoxy printed circuit coil having six turns on each side of the neck of the tube 12. The auxiliary horizontal deflection coil 59 superimposes a horizontal deflection on the electron beam 14 in addition to that imposed by the main hon'zontal deflection coil 18.

OPERATION To produce small point size characters in the electronic photocomposing system 10, the position P is selected in the potentiometer 36 and switch 43. This causes a control-biasing signal to be applied through the switch 43 from the processor and controller 40 to bias the modulator 54 such as at the point 60 in the input-output characteristic 56 shown in FIG. 4. Such a biasing point 60 causes the oscillatory signal 62 to be suppressed in the modulator 54. Hence no oscillations are applied to the auxiliary deflection coil 59 and the horizontal deflection of the electron beam 14 is invariable during each vertical sweep. The scanning spot 21 makes a trace on the face 24 of the tube 12 similar to the trace 64 in FIG. 2b. Hence for a small point size, each scanline is unidirectional.

When the point size is enlarged, the electronic data processor and controller 40 may, for example, bias the modulator 54 at the point 66 in the FIG. 4. In such a case the oscillatory signal 62 is modulated by the control-biasing signal to produce the output signal 68. The output oscillatory signal 68 is therefore applied to the auxiliary deflection coil 59 and consequently superimposes orthogonal alternations on each vertical sweep of the scanning beam 14 as shown in FIG. 312. Each scanline is therefore essentially bidirectional and the in terstices between character segments 30 are filled in, as shown in FIG. 3a. Hence uniformly dense characters are produced.

The oscillator 52 produces a sine wave oscillatory signal 62 as shown in FIG. 4. However the oscillator 52 may also be a trapezodial oscillator or any other alternating signal oscillator capable of filling in the interstices between adjacent scanlines. The frequency of the oscillator 52 is selected to be sufficiently high to cause the scanning spot 21 to overlap on each successive alternation to prevent white spots from occurring in a black segment.

Thus an electronic photocomposing system has been described that permits point sizes of characters to be changed in a simple manner without resulting in the graphic deterioration of the characters produced.

I claim:

1. In a system wherein light generated by a display device is utilized to create characters by a plurality of character slices, the combination comprising:

means for scanning said display device to provide a plurality oflinear scanlines in one direction;

means for providing successive positioning signals to step said one-directional scanlines orthogonally to produce a plurality of adjacent scanline segments that form said character slices;

means for providing a control signal for changing the point size of said characters; means for utilizing said control signal to change the lengths of said scanline segments and the spacing between adjacent scanlines, to create characters of said changed point size;

means providing a plurality of alternating signals;

means for modulating said alternating signals to produce output alternations having an amplitude based on the amount of modulation imposed by the modulating means;

means for utilizing said control signal to cause said modulating means to operate at a degree of modulation that corresponds to said changed point size; and

means for superimposing on said scanlines said output alternations to deflect said scanlines substantially orthogonal to said one direction so as to cause a bridging over the interstitial spaces between adjacent scanline segments.

2. A system inaccordance with claim 1 wherein said display device comprises a cathode ray tube.

3. A system in accordance with claim 2 wherein said cathode ray tube includes a scanning beam for producing light on the face of said tube.

4. A system in accordance with claim 3 wherein said cathode ray tube includes horizontal and vertical deflection circuits for deflecting said scanning beam over the face of said cathode ray tube.

5. A system in accordance with claim 4 wherein said vertical deflection circuit includes a sawtooth generator for deflecting said scanning beam in a substantially linear vertical sweep.

6. A system in accordance with claim 5 that further includes an auxiliary deflection coil that is coupled to said cathode ray tube and on which said alternations are superimposed to cause said orthogonal deflection of said scanning beam.

7. A system in accordance with claim 6 wherein said modulating means includes a modulator having a nonlinear inputoutput characteristic. 

1. In a system wherein light generated by a display device is utilized to create characters by a plurality of character slices, the combination comprising: means for scanning said display device to provide a plurality of linear scanlines in one direction; means for providing successive positioning signals to step said one-directional scanlines orthogonally to produce a plurality of adjacent scanline segments that form said character slices; means for providing a control signal for changing the point size of said characters; means for utilizing said control signal to change the lengths of said scanline segments and the spacing between adjacent scanlines, to create characters of said changed point size; means providing a plurality of alternating signals; means for modulating said alternating signals to produce output alternations having an amplitude based on the amount of modulation imposed by the modulating means; means for utilizing said control signal to cause said modulating means to operate at a degree of modulation that corresponds to said changed point size; and means for superimposing on said scanlines said output alternations to deflect said scanlines substantially orthogonal to said one direction so as to cause a bridging over the interstitial spaces between adjacent scanline segments.
 2. A system in accordance with claim 1 wherein said display device comprises a cathode ray tube.
 3. A system in accordance with claim 2 wherein said cathode ray tube includes a scanning beam for producing light on the face of said tube.
 4. A system in accordance with claim 3 wherein said cathode ray tube includes horizontal and vertical deflection circuits for deflecting said scanning beam over the face of said cathode ray tube.
 5. A system in accordance with claim 4 wherein said vertical deflection circuit includes a sawtooth generator for deflecting said scanning beam in a substantially linear vertical sweep.
 6. A system in accordance with claim 5 that further includes an auxiliary deflection coil that is coupled to said cathode ray tube and on which said alternations are superimposed to cause said orthogonal deflection of said scanning beam.
 7. A system in accordance with claim 6 wherein said modulating means includes a modulator having a nonlinear input-output characteristic. 